Silicon carbide has been noticed as materials of electronic devices such as small size and high output power semiconductors since silicon carbide has a larger band gap than silicon and is excellent in dielectric breakdown strength, heat resistance and radiation resistance, and as materials of optical devices since silicon carbide is excellent in joinability with other compound semiconductors excellent in optical characteristics. The silicon carbide single crystal is advantageous over the polycrystal of silicon carbide in that it is excellent in uniformity of intra-wafer characteristics when applied to devices such as wafers.
The wafer should be epitaxially grown as a thin film when luminescent devices and electronic devices are manufactured using the silicon carbide single crystal wafer. For example, silicon carbide is grown by a CVD method at a temperature range as high as from 1700 to 1800° C. or higher for epitaxially growing α-silicon carbide on (0001)c plane of α-silicon carbide single crystal. This is because planarity of the grown surface is remarkably impaired due to generation of triangular-pit defects when silicon carbide is grown at a temperature lower than 1700° C. However, exhaustion of members such as susceptors for holding the wafer is vigorous when silicon carbide is grown at a temperature range from 1700 to 1800° C. or higher, and electrical characteristics of the epitaxially grown film tend to be deteriorated due to impurities generated from the exhausted member.
Epitaxial growth without any dislocations is possible even at a temperature range from 1500 to 1600° C. by using a wafer having a so-called off-angle that is inclined several degrees to the (0001)c plane of the wafer. However, utilization rate of the bulk single crystal is remarkably decreased when the off-angle is large, and this problem is significant as the aperture size is larger. For example, when the wafer is manufactured by providing an off-angle of 3.5° that is common in silicon carbide wafer with crystal polymorph of 6H from a bulk single crystal grown in the [0001]c axis direction and having a crystal diameter of 50 mm and a crystal height of 20 mm, the utilization ratio of the bulk crystal is 84% and 16% of the crystal remains unused. The utilization ratio further decreases to 69% when the crystal diameter is expanded to 100 mm, and this decrease is not preferable since the price of the wafer increases. Since the dislocation density on the c-plane as a growth surface increases when the off-angle is large, characteristics of the element may be deteriorated. In addition, a large off-angle increases propagation of crystal defects of the substrate, especially surface dislocations, to an epitaxial growth layer, which increases a surface dislocation density in a grown crystal, thereby causing a problem of deterioration of characteristics of the element. In addition, when the off-angle is large, separation of the manufactured element by cleavage is difficult. This problem is crucial to optical devices, such as laser diodes, which utilize a cleavage surface itself.
While several technologies have been proposed as means for solving the above-mentioned problem, there is some room for improving the utilization rate of the bulk silicon carbide single crystal and for improving the characteristics of the element (for example, see Patent Document 1).
Patent Document 1: U.S. Pat. No. 4,912,064